Recently, there has been considerable interest in developing analytical techniques which utilize thermal-wave microscopy. In thermal-wave microscopy, a periodic heat source is focused on the surface of a sample. The heat source is typically supplied by either an intensity modulated laser beam, or a stream of particles such as an electron beam. If the sample absorbs the incident energy at or near the sample surface, a periodic surface heating results which in turn generates thermal waves that propagate from the irradiated spot. These thermal waves have the same frequency as the beam modulation frequency. The wavelength of the thermal waves is determined both by the frequency of the beam and by the thermal parameters of the sample.
In a thermal-wave microscope, thermal features beneath the sample surface are detected and imaged by sensing the thermal waves that scatter and reflect from these features. Thermal features are those regions of an otherwise homogeneous material that exhibit variations relative to their surroundings in thermal conductivity, thermal expansion coefficient or volume specific heat. Variations in these thermal parameters can arise from changes in basic material composition or from the presence of mechanical defects such cracks, voids and delaminations. Variations in thermal parameters can also arise from changes in the crystalline order or structure or due to the presence of small concentrations of foreign ions or lattice defects in an otherwise perfect crystal. The thermal waves are highly damped such that they travel only one or two wavelengths before becoming too weak to detect. Nevertheless, a variety of methods have been developed capable of sensing and measuring the thermal waves generated in a sample.
One method of detection includes the sensing of acoustic waves which are generated by the thermal waves. More particularly, acoustic waves are generated because thermal waves induce stress-strain oscillations in the heated region of the sample. These elastic waves are true propagating waves and can be detected with conventional ultrasonic transducers. It is believed that this technique, called thermoacoustic microscopy, was first disclosed in applicant's prior U.S. Pat. No. 4,255,971, issued Mar. 17, 1981, which is incorporated herein by reference.
While detection of thermal waves through the measurement of acoustic signals is effective, particularly at high frequency, it has certain disadvantages. For example, it is a "contact" technique requiring the attachment of an ultrasonic transducer to a sample. The latter requirement is time consuming and may not be suitable for high volume production situations. In addition, the acoustic signals may be influenced by other elastic effects such as plate-mode resonances. The detection of plate-mode resonances can be highly desirable when the goal is to evaluate the quality of the bond between two materials. (See, Applicant's co-pending Application, U.S. Ser. No. 381,891 filed May 25, 1982 and now U.S. Pat. No. 4,484,820 ). However, in other measurement situations, the various elastic and acoustic effects will complicate the thermal-wave analysis.
Accordingly, other methods are known in the prior art for detecting thermal waves which do not require any contact with the sample. For example, thermal-wave imaging can be performed by monitoring the local periodic temperature at the surface of the sample. One way of measuring the periodic temperature variations is through gas-microphone photoacoustics. In this technique, a sample is placed inside a closed chamber containing air and a sensitive microphone. Periodic conduction of heat from the sample surface to the air in the chamber gives rise to periodic pressure changes in the chamber. The periodic pressure changes can be detected by the microphone. (See, "Scanning Photo-Acoustic Microscopy (SPAM)", Wong, Scanned Image Microscopy, Academic Press, London, 1980).
Another method for measuring the periodic temperature changes at the surface of a sample includes the monitoring of a laser which traverses a gas or liquid medium that is in contact with the heated spot on the sample surface. The laser beam will undergo periodic deflections because of the periodic heat flow from the sample to the adjacent medium. (See, "The Mirage Effect in Photothermal Imaging", Fournier and Boccara, Scanned Image Microscopy, Academic Press, London, 1980). A third technique for measuring oscillating surface temperatures utilizes an infra-red detector that senses the periodic infra-red emission from the heated spot on the surface of the sample. (See, "Photoacoustic Microscopy at Low Modulation Frequencies", Luukkala, Scanned Image Microscopy, Academic Press, London, 1980).
The above-discussed techniques for measuring the periodic variations in the surface temperature of a sample provide adequate sensitivity at low modulation frequencies, for example, less than 10 kHz. As in all imaging systems employing waves, the resolution which can be obtained in a thermal-wave imaging system is dependent upon the wavelength of the imaging waves. Therefore, to obtain resolution in the micron and submicron range, it is necessary to use thermal waves that have wavelengths in a comparable range. Thermal waves in the micron and submicron range require beam modulation frequencies in the megahertz range. However, as pointed out above, the latter surface temperature detection techniques do not have sufficient sensitivity to operate at megahertz frequencies thereby precluding the imaging of micron and submicron features in a sample. Accordingly, it would be desirable to provide a detection system which could provide the desired sensitivity.
Recently, a measurement technique has been disclosed wherein local surface temperature fluctuations are measured using a laser interferometry system. This system depends upon the fact that the local temperature oscillations induced in a sample give rise to local vertical surface displacements caused by the expansion and contraction of the sample surface. A suitably designed laser interferometer could potentially detect surface temperature fluctuations at frequencies greater than 10 kHz. (See, "Photo Displacement Imaging", Ameri et al., Photoacoustics Spectroscopy Meeting, Technical Digest, paper THA6-1, Optical Society of America, 1981). However, it appears that even the disclosed interferometer system is not sufficiently sensitive to permit the imaging of micron size features. In addition, the development of interferometer systems may be restricted due to ancillary effects, such as changes in surface reflectivity caused by temperature oscillations or noise introduced by mechanical vibrations.
Accordingly, it is an object of the subject invention to provide a new and improved method for detecting thermal waves generated in a sample.
It is another object of the subject invention to provide a new and improved method for detecting thermal waves utilizing a laser probe.
It is a further object of the subject invention to provide a new and improved method for detecting thermal waves which is operative at frequencies in the megahertz range for imaging micron and submicron features.
It is still another object of the subject invention to provide a new and improved method for detecting thermal waves which does not require the attachment of an ultrasonic transducer in contact with the sample.
It is still a further object of the subject invention to provide a new and improved method for measuring thermal waves which is free from any elastic or acoustic effects present in thermoacoustic measurements.